A heat-sensitive recording material is relatively inexpensive because printed images can be obtained by simply heating it for allowing reaction between a leuco dye and a color developer. Also, recording devices for forming printed images from the heat-sensitive recording material are compact and easy maintenance can be used for the devices. From these advantages, a heat-sensitive recording material has been used as recording media for facsimile and printers of scientific measuring instruments, and various calculators. In recent years, a heat-sensitive recording material has become used as recording media such as POS labels, various tickets, lottery, and output media for automatic ticket vending-machines (ATM), CAD, handy-terminal and so on. Under the circumstances, the printed images are severely required to exhibit clearness when developed, and secure the clearness thereof for a long term.
Japanese Unexamined Patent Application Publication No. Hei 8-333329 (D1) proposes a crosslinked diphenylsulfone-based compound having a specific chemical formula, as a color stabilizer serving for preservation stability of a color image, particularly superior plasticizer resistance. Japanese Unexamined Patent Application Publication No. Hei 10-297090 (D2) discloses that use of the aforementioned crosslinked diphenylsulfone-based compound in combination with a hydroxydiphenylsulfone derivative is advantageous in improving dynamic sensitivity of development and plasticizer resistance.
Japanese Unexamined Patent Application Publication No. Hei 10-297092 (D3) proposes that printed images having improved water resistance, plasticizer resistance, and preservability in a high-temperature condition or a high-temperature high-humidity condition can be obtained by containing the crosslinked diphenylsulfone-based compound shown in D1, as a preservability-improving agent, in a concentration of 1 to 30% by weight of the total solid content. Example 10 in D3 discloses combination of bis(3-allyl-4-hydroxylphenyl)sulfone as a color developer, and 1,2-di(3-methylphenoxy)ethane as a sensitizer.
Japanese Unexamined Patent Application Publication No. 2001-80219 (D4) discloses a heat-sensitive recording material exhibiting good resistances to plasticizer, oil and humidity/heat (image preservability under 30° C., 80% RH) with respect to its printed images. The heat-sensitive recording material is produced by using the crosslinked diphenylsulfone-based compound shown in D1 as a color developer in combination with a graft copolymer of a starch and a polyvinyl acetate as an adhesive agent. D4 also recites that use of bis(3-allyl-4-hydroxyphenyl)sulfone as a color developer is advantageous in improving sensitivity of development, and preservability of printed images with time.
Japanese Unexamined Patent Application Publication No. 2004-276281 (D5) discloses that a heat-sensitive recording material with superior heat resistance and light resistance of unprinted portion, and superior plasticizer resistance and oil resistance of printed images is obtained by using a crosslinked diphenylsulfone-based compound and N-p-toluenesulfonyl-N′-3-(p-toluenesulfonyloxy)phenylurea as a color developer in combination with (3-(N-ethyl-p-toluidino)-6-methyl-7-anilinofuruorane) as a leuco dye. Comparative Example 4 in D5 discloses that oil resistance and plasticizer resistance are also exhibited by using bis(3-allyl-4-hydroxyphenyl) sulfone, in place of N-p-toluenesulfonyl-N′-3-(p-toluenesulfonyloxy)phenylurea.
In the heat-sensitive recording material, which is used as receipt in a POS (point of sales) system, various tickets, cash vouchers such as lottery tickets, or like sheets, improved plasticizer resistance, oil resistance, humidity/heat resistance of printed images are required. Furthermore, there is a demand for preserving printed images against daily household goods such as foods or drugs e.g. hand cream, milk, or vinegar. Among the foods or drugs, milk acts differently on printed images, unlike an oil or water, because milk is a mixture containing a milk fat, a protein, water, and the like. Owing to the complex composition of milk, even if printed images of a heat-sensitive recording material have sufficient oil resistance and/or water resistance, there are some cases that the printed images spilt with milk is likely to fade or discolor.
It is possible to secure preservability of printed images by increasing an initial printed density. However, in the case that printed images are barcode, for example a cash voucher or a like sheet, if an initial printed density is increased simply by increasing sensitivity of development, clearness of an edge of a thin line of the barcode may be impaired. This may cause another problem that desirable barcode printability may not be secured.
As described above, in recent years, there is a demand for securing milk resistance of printed images, without impairing barcode printability. Unfortunately, the conventional technology aiming at improving oil resistance and plasticizer resistance is not sufficient to satisfy the demand.